(General Background Art)
Active matrix liquid crystal display devices using thin-film transistors (hereinafter also referred to as TFTs) as switching elements are used widely in various fields including notebook personal computers, large-sized monitors for desk-top personal computers, mobile data terminals, the display panels of digital video cameras, and liquid crystal televisions.
As conceptually shown in (1, 1) and (1, 2) of FIG. 1, a liquid crystal display device changes a state in which liquid crystal molecules 40 are oriented with impression of a specified voltage on electrodes located over and under a liquid crystal layer on a pixel-by-pixel basis, thereby changes the light transmittance of each of pixels, and resultantly displays an image. In the drawing, 90 and 91 denote the respective electrodes formed on upper and lower substrates, 93 denotes a polarizing plate, and 94 denotes a polarizing plate or a reflecting plate. If the impression of the voltage on the electrodes is for so-called dc driving in which the direction of an electric field is constantly the same, i.e., either the upper or lower electrode is used constantly as a positive or negative electrode when the pixel is in the state which allows (or does not allow) passage of light as shown in (1, 1) and (1, 2), positive or negative impurity ions in a liquid crystal layer are attracted to the negative or positive electrode so that the distribution of the electric field between the pixel electrodes and the common electrode changes. This makes it difficult to apply a proper electric field to the liquid crystal molecules and causes a problem in displaying a clear image.
There are also cases where the liquid crystal material is electrolyzed or degraded. The attraction of impurity ions 45 to the upper and lower electrodes is conceptually shown in (1, 2) of FIG. 1.
When the pixel is in the state which allows (or does not allow) passage of light as shown in (2, 1) and (2, 2) of FIG. 1, so-called ac driving is normally performed in which the direction of the electric field and the direction in which the liquid crystal molecules are arranged are inverted at specified intervals, i.e., the polarities of the upper and lower electrodes are inverted at specified intervals. An example of a circuit for the pixel to be used for this purpose is shown in (3, 1) and (3, 2) of FIG. 1.
The simplest scheme of ac driving is so-called frame inversion driving in which each of the pixels over the entire display surface when it is in the state which allows (or does not allow) passage of light has a positive upper electrode (and a negative lower electrode) in a given display period and has a negative upper electrode in the subsequent display period. In short, the frame inversion driving scheme applies signals of the same polarity to each of the pixels over the entire display surface and inverts the polarity on a per display period basis.
However, an actual liquid crystal panel is asymmetrical because of the TFTs provided as the switching elements therein so that display brightness slightly differs depending on the positive or negative polarity of the impressed voltage. In the frame inversion driving scheme in which the signal of the same polarity is written in each of the pixels over the entire display surface, therefore, the difference in brightness between a frame in which a positive signal is applied and a frame in which a negative signal is applied is observed as flicker. It is to be noted that the structure of the pixel, the motion of the liquid crystal molecules, the circuit for inversion, or the like in the liquid crystal device shown in FIG. 1 is strictly exemplary or conceptual and there are other types of variations. However, the basic items of frame inversion driving, control of light transmission using the motion or tilt of the liquid crystal molecules responsive to the electric field, and the like are the same.
To eliminate flicker, there is a method in which pixels with reversed polarities are alternately arranged in two dimensions so that brightness is averaged. As examples of the method, there can be listed image-signal-line inversion driving which changes the polarity on a per image-signal-line basis, scan-signal-line inversion driving which performs polarity inversion on a per scan-signal-line basis, a combination of the image-signal-line inversion driving and the scan-signal-line inversion driving, and dot inversion driving which inverts the polarity of an electric field applied to pixels adjoining longitudinally and laterally in a screen on a per pixel basis.
Of the foregoing methods, the dot inversion driving has the advantages of unobtrusive flicker and display with a uniform brightness distribution since the pixels of positive and negative polarities are arranged in a checkered pattern. However, the dot inversion driving has the disadvantages of increased power consumption and increased charging load on a driving IC since the polarities are inverted in each of the rows and columns and therefore a driving waveform presents an increased number of voltage inversions during the transfer of a signal voltage to the driving IC and during the charging of the pixel with an image signal. To eliminate the disadvantages, Japanese Unexamined Patent Publication No. HEI 4-223428 discloses pseudo dot inversion driving which provides dot inversion display as a polarity pattern on a screen, while performing the image-signal-line inversion driving and the scan-signal-line inversion driving in terms of electric signals. The pseudo dot inversion driving aims at highly uniform display, similarly to the dot inversion driving, but with a simpler driving waveform than used by the dot inversion driving.
In (1) and (2) of FIG. 2 are shown two equivalent circuits for a liquid crystal display device, which are for performing pseudo dot inversion driving and disclosed in the foregoing publication. In each of the circuits, a TFT 1 is disposed in the vicinity of each of the points of intersection of a plurality of image signal lines 2 and intersecting scan signal lines 3 in such a manner that connection is provided between the source and gate electrodes of the TFT 1. The drain electrode of the TFT 1 is connected to a liquid crystal layer 4 and to accumulated capacitance 5 in parallel with the liquid crystal layer 4.
In (1) of FIG. 2, the TFTs 1 of two longitudinally adjoining pixels along the image signal lines 2 have respective source electrodes connected to the different image signal lines 2. If the liquid crystal panel is driven in the image-signal-line inversion scheme, the polarities of voltages impressed on the individual pixels are inverted as shown in FIG. 3 so that dot inversion display is achieved.
On the other hand, (2) of FIG. 2 illustrates another method in which the TFTs 1 of laterally adjoining pixels along the scan signal lines 3 have respective gate electrodes connected to the different scan signal lines 3. In the liquid crystal panel, the polarities of voltages impressed on the adjacent pixels are inverted by the scan-signal-line inversion driving so that dot inversion display is achieved.
Thus, dot inversion display is achievable by performing the image-signal-line inversion driving in the liquid crystal display panel in which the source electrodes of the adjoining TFTs in the region enclosed by the adjacent two image signal lines are connected to the different image signal lines or in the liquid crystal display panel in which the gate electrodes of the adjoining TFTs in the region enclosed by the adjacent two scan signal lines are connected to the different image signal lines.
In either case, the polarity of the voltage impressed on each of the pixels is inverted in the subsequent frame period such that an ac voltage is impressed on each of the pixels.
(Background Art Viewed from Problems to be Solved by the Invention)
During the fabrication of a liquid crystal display panel, layers including a metal film, a semiconductor layer, and an insulating layer are deposited (formed) a plurality of times and the total of five to eight photolithographic steps are normally performed after the deposition of each layer or after the deposition of given materials so as to pattern the individual layers (the process of removing unwanted portions and regions of the layers composed of the deposited given materials by dry etching or the like and leaving only the required portions), whereby TFTs, pixels, and the like are formed. When the photolithographic steps are performed, alignment is effected between a substrate and a photo mask. However, an alignment shift (misalignment) of about one micrometer to several micrometers does occur depending on conditions including the sizes of the substrate and the display surface.
FIGS. 4 and 5 are views for illustrating the influence of the shift in a conventional TFT. It is to be noted that the depiction of a distinct boundary between insulating and protective films may be omitted in the subsequent plan views. In the steps of fabricating a TFT, the source and drain regions are typically formed by simultaneously pattering metal in a single layer. For the sake of clarity, the drawings show an exemplary case where the source and drain electrodes are shifted only in a direction parallel to the scan signal lines. Problems occurring in the case of a shift in an orthogonal direction are negligible, though they differ depending on the configuration and size of the gate electrode.
In each of FIGS. 4 and 5, each of the TFTs 1 is composed of: a gate electrode 11; a source electrode 7, 71, or 72; a drain electrode 8, 81, or 82; and a channel protective film 14 and formed in the vicinity of each of the points of intersection of the image signal lines 2 and the scan signal lines 3. The gate electrodes 11 are connected to the scan signal lines and the source electrodes 12 are connected to the image signal lines. The drain electrodes 8, 81, and 82 are connected to the pixel electrodes 6. Although the relative sizes of the pixel electrodes are larger, they are depicted narrower and smaller since they are not directly relevant to the spirit of the invention.
A cross-sectional view of the portion of the TFT is shown in the lower part of FIG. 4. In the drawing, 9 denotes a substrate, 89 denotes a contact hole for the drain electrode, 79 denotes a contact hole for the source electrode, and 141 denotes a gate insulating film. As can be clearly seen from the cross-sectional view, metal films 82 and 72 forming the source and drain electrodes are opposed to the semiconductor layer 15 with a channel protective film 142 interposed therebetween and capacitances are formed at the overlapping portions (the hatched and dotted portions in the drawing) when viewed from above an upper surface orthogonal to the substrate surface.
In FIG. 5, an electrode area occupied by the overlapping portion between the channel protective film 14 and source electrode of the TFT is designated at Ss and an electrode area occupied by the overlapping portion between the channel protective film 14 and drain electrode of the TFT is designated at Sd. In the TFT shown in (1) of FIG. 5 which is free from an alignment shift, Ss=Sd is satisfied. In the case where a shift occurs in the direction in which the overlapping region between the source electrode 71 and the channel protective film 14 increases as in (2) of FIG. 5 (rightward in the drawing), Ss>Sd is satisfied. In the case where a shift occurs in the direction in which the overlapping region between the source electrode 72 and the channel protective film 14 decreases as in (3) of FIG. 5 (leftward in the drawing), Ss<Sd is satisfied conversely. In short, an ability difference is produced between the TFTs depending on the direction of a shift.
FIG. 4 shows the case where the source and drain electrodes of the TFT structure for performing the pseudo dot inversion driving by the image-signal-line inversion driving using the equivalent circuit configuration of (1) of FIG. 2 are displaced rightward relative to the scan signal lines.
In the structure shown in the drawing, the source electrodes of the longitudinally adjoining TFTs which are interposed between the two image signal lines are connected to the different image signal lines. Specifically, one TFT 101 of the two TFTs interposed between the two adjacent image signal lines 21 and 22 has the source electrode 71 connected to the image signal line 21 and the other TFT 102 has the source electrode 72 connected to the image signal line 22 other than the image signal line 21.
In the structure, the rightward displacement of the source and drain electrodes increases the overlapping region between the source electrode 71 and the channel protective film in the upper TFT 101, while it decreases the overlapping region between the source electrode 72 and the channel protective film in the lower TFT 102. Accordingly, Ss>Sd is satisfied in the TFT 101 connected to the image signal line 21, while Ss<Sd is conversely satisfied in the TFT 102 connected to the image signal line 22.
Thus, the capacitance between the source and gate electrodes of the TFT and the capacitance between the drain and gate electrodes of the TFT differ from one scan signal line to another due to an alignment shift during the fabrication of the TFT. This causes a difference between the charging abilities of the adjacent pixels and non-uniform display such as flicker or a vertical/horizontal string.
The occurrence of the foregoing problems is not limited to the channel-protective-film bottom-gate thin film transistor. FIG. 6 shows the case of a thin-film transistor of another type. In (1) of FIG. 6 is shown the case of a top-gate thin-film transistor. In (1) of FIG. 6, the distance LSG between the gate electrode 11 and the source electrode 7 is different from the distance LDG between the gate electrode 11 and the drain electrode 8. In addition, the distance LSLDD between the LDD region 151 and the contact hole 79 for the source electrode is also different from the distance LDLDD between the LDD region 151 and the contact hole 89 for the drain electrode. Although the gate insulating film 141 and the channel protective film 142 are formed in (1) of FIG. 6, the latter may not be formed in some cases.
In (2) of FIG. 6 is shown a channel-etched bottom-gate thin-film transistor, in which the overlapping portion between the source electrode 7 and the semiconductor layer 15 is larger than the overlapping portion between the drain electrode 8 and the semiconductor layer.
In (3) of FIG. 6 is shown the case of the channel-etched bottom-gate thin-film transistor in which an alignment shift has occurred between the gate electrode 11 and the semiconductor layer 15. Although thin-film transistors other than the foregoing include one in which, e.g., the length of the gate electrode in the channel direction (width) is smaller than that of the semiconductor layer, the occurrence of the foregoing problems cannot be circumvented in any type.
The occurrence of the problems is not limited to a thin-film transistor. Similar problems may also occur in a diode, which are illustrated by using FIG. 7. In (1) of FIG. 7 is shown a plan view of one pixel in a liquid crystal display device using a diode. In the drawing, 111 denotes a diode, 6 denotes a pixel electrode, 2 denotes a counter electrode, 3 denotes a first electrode, 60 denotes a metal layer (end portion) connected to the pixel electrode, and 14 and 15 denote an insulating film and a semiconductor layer.
A driving method will be described for reference purposes. In accordance with the method, a scan signal for the diode is inputted to a scan line. The diode to which the ON signal has been inputted is turned ON and the pixel electrode 6 has the same potential as the ON voltage for the diode. The difference between an image signal applied to an image signal time and the pixel potential is stored in a liquid crystal layer. The diode is turned OFF during the scanning of the subsequent scan line and the voltage impressed in the ON state is held, whereby display and the like are effected.
If the end portion 60 of the pixel electrode is covered completely with the first electrode 99 as shown in (2) of FIG. 7, an alignment shift is irrelevant. In reality, however, the end portion 60 is covered halfway as shown in (3) of FIG. 7, so that a variation occurs in the capacitance of the portion encircled by the symbol ◯.
Accordingly, the development of technology has been desired which allows thin-film transistors arranged over the entire surface of a liquid crystal display panel for particularly performing pseudo dot inversion driving to have equal charging abilities even if an alignment shift occurs between the individual layers.